The present invention relates to a multi-layered light diffusion plate and to a liquid crystal display device comprising the same, and, more particularly, to a multi-layered light diffusion plate, which is manufactured by coextruding a transparent layer, including an amorphous transparent thermoplastic resin, and a light diffusion layer, including an amorphous transparent thermoplastic resin and transparent particles, so that the manufacturing process thereof is simple and efficient, and which has excellent light transmissivity and light diffusivity, high brightness, and high brightness uniformity, thereby improving viewing angle characteristics, and to a liquid crystal display device comprising the same.
Generally, in a liquid crystal display device, such as a monitor or a liquid crystal TV, a light diffusion sheet has been used to form a point light source or a line light source into a surface light source. This light diffusion sheet must have high light diffusivity in order to prevent light sources or internal objects, disposed posterior thereto, from being seen from the exterior and in order to show uniform luminosity, and must have high light transmissivity in order to increase light efficiency, that is, high brightness and low power consumption. Furthermore, this light diffusion sheet is required to have high brightness uniformity in order to secure wide viewing angles.
Light diffusion sheets, which have been developed up to now, areas follows: (1) a light diffusion sheet obtained by extruding a transparent thermoplastic resin into a sheet shape, and then physically forming protruded portions on the surfaces of the sheet (Japanese Unexamined Patent Application Publication No. Hei04-275501); (2) a light diffusion sheet or film obtained by layering a light diffusion layer, formed of a transparent resin including particles, on a transparent base film, such as a polyester resin (Japanese Unexamined Patent Application Publication No. Hei06-059108); (3) a light diffusion sheet obtained by mixing inorganic particles in a transparent resin to form a mixture, and then forming the mixture into a sheet having light diffusivity (Japanese Unexamined Patent Application Publication No. Hei06-347617); (4) an oriented polyester film, having excellent light diffusivity, comprising at least two layers produced through a coextrusion process (Japanese Unexamined Patent Application Publication No. 2001-322218 and Korean Unexamined Patent Application Publication No. 2002-48116); and (5) a light diffusion sheet obtained by mixing beads in a melted transparent resin to form a mixture, and then extruding the mixture (Japanese Unexamined Patent Application Publication No. Hei06-123802).
The light diffusion sheets mentioned above in (1) and (2) are surface light diffusive sheets obtaining light diffusivity by forming protruded portions on the surfaces thereof or layering a light diffusion layer on the surfaces thereof. However, in light diffusion sheets in general, and the light diffusion sheets (1) and (2), in particular, when the light diffusivity is obtained through a surface treatment, there is a strong likelihood that the surfaces of the sheet will be damaged upon treatment. Particularly, the light diffusion sheet (2) that is now generally used in a small-sized liquid crystal display of a notebook computer, etc. has a problem in that it is difficult to form a layered structure, which is required in order to realize a high-performance liquid crystal display device and a multi-functional liquid crystal display device. Further, this light diffusion sheet has a problem in that a light diffusion layer applied on the surface thereof is easily peeled off due to the difference in the thermal expansion rate between the base film and the sheet.
Meanwhile, the light diffusion sheets mentioned above in (3) and (5) have advantages in that sheets are imparted with light diffusivity while the sheets are formed, thereby obtaining a high light diffusivity. However, the light diffusion sheet (3) has disadvantages in that, because inorganic particles are added to impart light diffusivity, light transmissivity is decreased, and therefore, the brightness and mechanical properties are greatly decreased. Further, this sheet also has a problem in that a processing machine becomes worn down when the sheets are processed for a long time. In the light diffusion sheets (4), since a crystalline polyester resin is used to manufacture the sheet, in order to obtain a light diffusion sheet having a desired light diffusivity and total light transmissivity, a process of manufacturing the light diffusion sheet must include the steps of preparing a non-stretched polyester sheet including at least two layers, stretching the non-stretched polyester sheet by a predetermined elongation percentage in a vertical or horizontal direction at a predetermined temperature, and heat-treating the stretched polyester sheet. In this case, when the process of stretching the non-stretched polyester sheet is not performed, transparency and optical characteristics are greatly decreased due to the presence of crystals between polymer chains formed in a process of cooling the sheet. Accordingly, there are disadvantages in that the manufacturing process thereof is complex and the productivity thereof is decreased, because the stretching process is always required in order to secure the transparency and optical characteristics. The light diffusion sheet (5) mentioned above is now most widely used, but has a problem in that it exhibits neither the brightness nor the brightness uniformity sufficient for application to liquid crystal panels having increasingly wide viewing angles, which have been developed recently.